Oriented thermoplastic monofilaments have long been used in a variety of applications, including, for example, tire cord and fishline. Typically, the monofilament is extruded, quenched, and oriented by drawing in one or more stages to improve its tensile properties.
Other techniques have been previously developed to further improve the performance characteristics of oriented thermoplastic monofilaments. High knot strength polyamide monofilaments can be prepared using high pressure steam, as disclosed in Boyer, et al., U.S. Pat. No. 3,063,189. Gauntt, in U.S. Pat. Nos. 4,056,652 and 4,009,511 discloses treatment with high pressure steam in the first drawing stage of monofilament production, and Saito, et.al., in U.S. Pat. No. 4,338,227, discloses treatment with high pressure steam in the second drawing stage. These patents generally teach that treatment of the filament surface with high pressure steam can increase the knot strength by deorienting the surface of the filament.
A major disadvantage of these techniques is that they require a high pressure steam enclosure to operate. The high pressure steam enclosures are costly to construct and operate. Also, when filament breaks occur, restringing is time-consuming and therefore costly because the steam pressure must be relieved and the high pressure enclosure opened. In addition, for a fishline, the highest possible transparency is desireable.
Despite the known drawbacks of steam treatment, its use has continued for the processing of thermoplastic monofilament, since the steam treatment accomplished the desired property improvement without seriously depreciating the other desireable performance characteristics.